Conventional electronic control units for vehicle's engine control are provided with a main power-supply circuit designed to output a constant power supply voltage Vm based on battery power when an ignition switch is turned on. The electronic control units also provided with a sub power-supply circuit designed to continuously output a constant power supply voltage Vs.
The power-supply voltage Vm output from the main power-supply circuit is supplied to electrical devices, such as a microcomputer and the like. The power supply voltage Vs output from the sub power-supply circuit is supplied to circuits and memories, such as, backup RAMs (Random Access Memories); these circuits and memories are required to continuously operate with remarkably low power consumption as compared with the microcomputer.
As an example of electronic control units having such a structure set forth above, an electronic control unit, referred to simply as “ECU”, is provided with a timer operating based on the power supply voltage output from the sub power-supply circuit. The timer, normally so called soak timer, is configured to measure a standby (suspend) period of the microcomputer, in other words, a period during power supply interruption from the main power-supply circuit to the microcomputer. The timer is configured to cause the main power-supply circuit to output the power supply voltage Vm to the microcomputer to activate it when the measured period reaches a predetermined setting period. The electronic control unit with the timer set forth above is typically disclosed in U.S. Patent Application No. 2003/0093189A1 corresponding to Japanese Unexamined Patent Publication No. 2003-139874.
Install of such a timer (soak timer) into the ECU allows the microcomputer to perform desired tasks when the predetermined setting period has elapsed after turning-off of the ignition switch without continuous supply of the power supply voltage to the microcomputer. This makes it possible to reduce the ECU's power consumption.
Specifically, the ECU, typically disclosed in the U.S. patent application, includes (a) the main power-supply circuit and (b) the timer.
The main power-supply circuit is configured to output the power supply voltage Vm when any one of a switch signal in response to turning on or off of the ignition switch and an activate signal created inside the ECU has an active level.
The timer is integrated with a counter whose count value is resettable by the microcomputer. The counter is operative to count (count up) from its initial value when the ignition switch is turned off so that the power supply voltage Vm is interrupted from the main power supply-circuit to the microcomputer. The timer is configured to turn the activate signal from an inactive level to an active level to allow the main power-supply circuit to output the power supply voltage Vm to the microcomputer, thereby activating it.
Note that the described ECU with the timer allows diagnosis of an evaporative emission control system whose structure is typically disclosed in the U.S. patent application.
Specifically, in check of an evaporative emission control system of this type, while a system for collecting fuel evaporative emissions escaping from the fuel tank is closed, pressurization or reduction in the system to create variation in pressure in the evaporative emission control system allows air-tightness in the system to be checked. Immediately after the engine has been operated for a long period under high-load conditions, it is difficult to obtain an accurate result of the check because the fuel in the fuel tank easily evaporates.
Accordingly, after a constant period has elapsed from stop of the engine, such as turning-off of the ignition switch, the timer causes the microcomputer to boot up so that microcomputer checks air-tightness in the evaporative emission control system set forth above.
In such an ECU used to check air-tightness in the evaporative emission control system, a failure or malfunction of the timer may make it difficult for the microcomputer to perform predetermined tasks within off state of the ignition switch; these predetermined tasks include air-tightness checking operations set forth above.
In order to solve the difficulty in performing the predetermined tasks, when booting up in response to turning on of the ignition switch, the microcomputer of the ECU disclosed in the U.S. patent application reads out a count value of the counter of the timer and determines whether the timer properly operates based on the readout count value.
The techniques disclosed in the U.S. patent application are to detect an abnormality representing that the timer cannot activate the microcomputer, in other words, the predetermined tasks cannot be performed, within off state of the ignition switch after turning on of the ignition switch. The techniques are therefore not to detect such an abnormality within off state of the ignition switch.
In a case where a failure representing that the timer keeps the activate level of the activate signal being sent to the main power-supply circuit, even if the ignition switch is turned off, the power supply voltage remains fed to the microcomputer from the main power-supply circuit. There have been requests for detecting such a timer failure.